[Docs][Release] Update post release script to move stable docs to versioned + keep dev docs

[asfimport]

xref apache/arrow-site#187

We need to update the post-09-docs.sh script to keep the dev docs and to move the current stable docs to a versioned sub-directory

Reporter: Joris Van den Bossche / @jorisvandenbossche
Assignee: Joris Van den Bossche / @jorisvandenbossche

PRs and other links:

• GitHub Pull Request #12355

_{Note: This issue was originally created as ARROW-15601. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 12355
#12355